Printed ink is subjected to abrasive forces from the moment it is applied onto a substrate, e.g. paper or other suitable medium. The damage caused by the mechanical forces is classified into various categories, mainly due to the diverse appearance of the damage. In one form of damage, the ink surface may be scratched and as a result the gloss is reduced. In a second form of damage, the ink layer may be removed due to deeper scratches. In a third form of damage, the printed ink can be partially or completely torn out due to poor adhesion to the substrate. Among other effects, mechanical forces may smear the ink on the substrate.
Various methods have been developed over the years to reduce the negative effect of mechanical forces acting on printed inks. The latter can be divided into three categories namely: preprinting treatment of the substrate, suitable ink formulation and post printing treatments. The printed ink layer may vary from 1 to several micron thicknesses per ink separation depending on the printing method. Relatively thin printed ink layers such as those formed in offset printing as well as liquid electrostatic printing (LEP) may be protected by more than one of the above-mentioned techniques.
Pre-printing techniques involve coating of the substrate with a binding layer for improvement of adhesion. However, pre-printing treatment cannot protect against print surface scratches. Corona treatment is widely used for synthetic substrates such a polypropylene or polyethylene. However, Corona treatments are, generally speaking, specific to a narrow selection of synthetic substrate. In addition, poisonous ozone can be formed by the corona treatment.
Pre-coating of the substrate with an adhesion promotion layer is another type of pre printing treatment which can involve an off line coating process. Printing companies typically use out sourced coating services. As a result the printer is compelled to keep stocks of coated substrate, which can increase production cost.
The addition of suitable additives into the ink formulation is another way for reinforcing the ink layer. Wetting agents as well as adhesion promoters may improve adhesion of the ink to the substrate. Slipping agents such as hard wax dispersed in the ink form under suitable condition a protective layer on the ink surface and may also reduce friction coefficient of the ink thin layer allowing other prints to glide on the ink surface instead of abrading it.
Crosslinkable acrylate groups incorporated into the ink ingredients e.g. in the form of suitable solvent monomers or prepolymers, may improve durability to some extent. Post-printing exposure to heat or UV radiation is used for drying and activation of crosslinkable functional groups.
It is well known that the effect of the above discussed additives is limited to particular inks and depends on the substrate. In many cases these additives introduce unwanted side effects. Wax, for example, can imparts slip properties to the printed layer, and, as a result, other prints slip on top of the printed image and thus, the printed ink is less damaged. On the other hand, the solid wax particles which are not inherently part of the ink film layer can introduce micro defects. These imperfections may increase susceptibility to scratching by sharp objects. Cross linking of functional groups formulated into the ink such as UV curing materials may not be favoured due to the relatively expensive curing systems and the high energy consumption of these systems.
Post-printing reinforcement of printed ink may include coating the printed ink with a transparent layer. The applied layer creates a physical barrier which protects the ink layer. The protecting layer such as overprint varnish may be applied in the printing process as an additional colour separation. Protective varnish can be applied also offline using a coating system. Alternatively, coating the print with a thin solid laminate is widely used in the printing industry.
In a typical liquid electrostatic printer, the various inks are printed by the same printing engine, each ink in a separate printing engine cycle. More inks including overprint varnish would prolong the printing process.
However, in the case of offset lithography each ink is printed by a different printing engine. The addition of an ink would require an additional printing engine. Offline coating of printed material either by a liquid or solid coating requires additional exclusive coating process and system. It is worth mentioning that offline coating of sheet-fed prints is more complex than coating prints in a web form.
It would be desirable to provide a technique for improving the durability of printed inks that is an alternative to the techniques of the prior art and ideally overcomes at least one problem associated with the techniques of the prior art, whether or not expressly mentioned herein.